1. Field of the Invention
The fixation of atmospheric nitrogen associated with specific legumes is the result of a highly specific symbiotic relationship with rhizobial bacteria. These indigenous bacteria dwell in the soil and are responsible for the formation of nodules in the roots of leguminous plants as sites for the nitrogen fixation. Currently, these rhizobia are classified by growth rate in free-living cultures, with the fast-growing organisms being designated as Rhizobium and the slow-growing organisms as Bradyrhizobium. Many of the rhizobial strains are not only host-specific but also differ with respect to capacity for effective symbiosis. Those strains which are able to infect a plurality of host plants across species or genus lines are said to be "cross-nodulating." Commercial inocula generally consist of a mixture of rhizobial strains to insure the widest potential for effective symbiosis within the appropriate crop. This invention therefore relates to the commercial practice of inoculating soybeans and other legume seedlings with specific rhizobia in order to increase effective nodulation and growth of leguminous crops.
1. Description of the Prior Art
In the art of soybean cultivation, unspecified strains of B. japonicum have become recognized as standard inoculants for initiating nodulation and nitrogen fixation. One strain of B. japonicum of interest is L-259 (USDA strain 26). When cultivated on artificial media, this strain in characterized by colorless colonies. Kaneshiro et al. [Current Microbiol. 8: 301-306 (1983)] report the discovery of tan-colored (tan), variant colonies which were selected and isolated from the colorless parent strain L-259 after enrichment and growth on a glutamate-limited medium supplemented with tryptophan (trp). These spontaneous variants retained the parental strain capacity for asymbiotic acetylene-reducing activity on a glutamate-mannitol-gluconate (GMG) medium; thereby indicating nitrogen-fixing capability. The variants were also reported to catabolize exogenous trp to over-produce indoleacetic acid (IAA), a plant auxin associated with new root formations in legumes, and indolepyruvic acid (IPA), a catabolic precursor to IAA. Kaneshiro speculated the significance of these variant strains as being useful in an assay for identifying nodule symbionts, though no in situ studies were reported.
R. lequminosarum [Wang et al., Planta 155: 345-349 (1982), and R. trifolii [Badenoch-Jones et al., Biomed. Mass Spec. 9: 429-437 (1982)] have been found to produce IAA and other indolic compounds without affecting legume nodulations significantly. These organisms in free-living cultures are estimated to produce indolic compounds in the range of less than 1 mg. per liter.